Lane: Yep, it's the same as what Kiriri said.
Though, without pool-manager it is possible to do it surprisingly easy with Playmaker but you kinda do have to set up a "enable/disable" system that controls the things such as the velocity, any particle effects, etc... with the shurikens, since they have a trail-renderer applied, i had to figure out how to adjust it and the proper timing (hint: next frame event is your lucky star in this to ensure that the events are firing off in the proper order)
it's also just as simple to tell it to spawn the appropriate number of items you want to store and if you use the parenting actions and the enable/disable system i mentioned, you don't have to worry about them flying around and mucking up things in your scene (though if you have them able to use a rigid-body on other instantiations of themselves, you might want to consider enclosing them in a box so that if they do fly around from the rigidbody interacting with others like themselves and their position is stored in the same space, that might help to keep them in one space... but, for what i used, i just told the physics engine to give them a separate layer (all projectiles) and ignore other projectiles. it's unlikely that you'd have players firing bullets and hitting other bullets anyway.)